The role of sedimentation history and lithology on fluid flow and reactions in off-axis hydrothermal systems: A perspective from the Troodos ophiolite

• Two lava sections from the Troodos ophiolite show contrasting enrichment in CO2, alkali elements, and Sr- and Li-isotopes.
• The two lava sections show contrasting calcite formation temperature depth profiles.
• Chemically enriched lava section records higher reactive fluid fluxes in a cold crustal aquifer.
• Off-axis hydrothermal systems have significant lateral variation in the extent of ocean–crust exchange and thermal regime.
• Sedimentation history and crustal architecture are important controls on off-axis hydrothermal fluxes.

Off-axis hydrothermal systems are thought to carry globally significant chemical fluxes but different ocean crust sections show widely differing extents of alteration making the quantification of these fluxes complex. With the aim of better understanding the origin of this diversity in alteration extent we have studied two sections of the lava pile in the Troodos ophiolite with distinct sedimentation history and volcanic architecture. The Akaki section is dominated by pillow lavas and the oldest sediments overlying the crust are ~ 20 Myr younger than the ophiolite. Here there is a 300 m thick zone at the top of the lavas that is enriched in CO2 and alkali elements, and has high 87Sr/86Sr and δ7Li. These features indicate extensive chemical exchange with seawater. In contrast to the Akaki area, the Onophrious section is dominated by sheet flows and the oldest sediments are of the same age as the ophiolite. Here the CO2 and alkali element enriched zone is much thinner (< 100 m), is less enriched in these elements (e.g. by a factor of three for CO2), and has lower 87Sr/86Sr and δ7Li. The O-isotopic compositions of calcites from these CO2- and alkali-enriched zones were precipitated from fluids with bottom water temperatures (~ 10 °C). Maintaining such low temperatures to 300 m depth in the crust in the Akaki area suggests that this was a region of recharge. Below these CO2- and alkali-enriched zones temperatures increase with depth such that calcite precipitation in the Onophrious area occurred at ~ 10 °C higher temperature, at any given depth, than in the Akaki area. The increase in precipitation temperature with depth indicates poor thermal mixing within the crustal aquifer, likely due to laterally continuous sheet flows restricting the permeability. The chemical and thermal constraints suggest that both timing of the onset of sedimentation and volcanic architecture play important roles in controlling fluid and chemical fluxes. The same signal can be seen in drill core data from the modern ocean basins, with higher sedimentation rates leading to lower fluid fluxes and higher temperatures in the crustal aquifer.